Can you add a third seat to an expedition that only came with the 2nd row seat?

Yes you can. Here is the best site that I know of to get the 3rd row. http://www.littlepassengerseats.com/ford_expedition.htm

Make sure it has the rear seat brackets to hold them in with.

what size seat belts do you need for the third row. the seat I got doesn't include them. any one know the measurements needed?

My 3rd row seats have the seatbelts already installed at the factory.